The present invention relates to a tool for assembling printed circuit boards having utility as a component mask for preserving unoccupied lead-holes in the printed circuit board during mass connection of components thereto, and its method of use.
Electronic circuitry often includes one or more printed circuit boards providing the physical and electrical substrates connecting various electronic components together. A printed circuit board generally is a sheet of a rigid non-conductive material having electrically conductive lead patterns printed or otherwise provided on one or both of its sides. The lead patterns connect various terminal lead-holes extending through the underlying board, and the desired circuit is formed by installing electrical and electronic components, such as integrated circuit chips, transisters, etc., on the board with their terminals protruding into and through the lead-holes. The terminals of the components are then attached to the printed circuit board by soldering or the like to provide both physical connection of the components to the board and electrical connection of their terminals to the conductive lead patterns.
The above procedure is only practical if most of the component terminals are secured to the printed circuit board simultaneously by a mass connecting technique, such as wave soldering. That is, it would be quite impractical to independently secure each of the separate component terminals to the board, in view of the large number of them and their small size. Thus, electronic circuits are generally formed on printed circuit boards by mounting the electronic components on a single side of the printed circuit board, with the terminals extending to the opposite side. Such opposite side then is subjected to a mass connecting technique to simultaneously secure the terminals of all such components to the board. Wave soldering, i.e., the subjection of the underneath side of such a printed circuit board to a traveling wave of solder, is quite widely used as a mass connection technique.
The difficulty with use of wave soldering is that it is incapable of discriminating between those lead-holes extending through a printed circuit board which in fact have component terminals extending therethrough, and those lead-holes which do not. And it is not unusual for it to be desirable to keep unobstructed of solder at least some of those lead-holes which are unoccupied by component terminals at the time of wave soldering. For example, for some circuitry it is desirable to be able to secure some electronic components to a board after most are already secured thereto by wave soldering. Moreover, it is not unusual for component parts to be unavailable due to supply shortages and the like, at the time it is desired that a board be subjected to wave soldering.
Various techniques and procedures have been used in the past to prevent solder from entering lead-holes which are unoccupied at the time the printed circuit board is subjected to wave soldering. For example, tape or latex has been applied over the ends of holes to be spared on the underneath side of the printed circuit board, to mask such holes from solder. Since it is difficult to apply such tape or latex to the underneath side of a board after electrical components have been placed into position on the upper side of such board, use of such masking techniques has been limited to use prior to component assembly. Thus, such techniques are unavailable for uses at the last moment, i.e., when some parts on order are not delivered on time. Moreover, use of a tape often leaves an undesired adhesive residue on the underneath side of the board. And if latex or the like is used, "plugs" of the material often remain in the lead-holes after the same is otherwise removed.
Another approach used in the past, has been to insert simple wooden toothpicks in the lead-holes to be preserved. This approach leaves much to be desired. The cross-sectional dimensions of toothpicks are not uniform, with the result that it is not unusual to find lead-holes which are partly plugged with solder because a toothpick which was meant to preserve the same did not extend completely through the hole. Toothpicks which are only loosely engaged within a hole have the disadvantage of either leaving too restricted of an unobstructed lead-hole, or falling out before or during wave soldering. The high temperature associated with wave soldering also chars toothpicks, leaving charred residue within the lead-holes. In another approach used in the past, a plurality of uniformly tapered metal pins embedded in a supporting block has been used to mask lead-holes during mass soldering of printed circuit boards. The pins are tapered so that they wedge against the edge of the lead-holes to hold the supporting block spaced away from the printed circuit board. The metal pins are constructed of a material that is thermally conductive while at the same time not being susceptible to wetting by most common fluxes found in solder used in mass soldering processes. These combined characteristics of the pin material prevents solder from entering the lead-holes. However, such non-solderable metal pins are susceptible to wetting by highly active fluxes. When wetted, solder is permitted to enter the lead-holes and adhere to the pins. This often results in the formation of solder obstructions in the lead-holes or prevention of the easy removal of the pins from the lead-holes after the completion of the wave soldering process. To avoid such adverse consequences, a coating of Teflon fluorocarbon polymer has been applied to the pins to prevent wetting by highly active fluxes. While the Teflon coating prevents such wetting, the pins remain thermally conductive to prevent any solder from entering the lead-holes.